Add automatic build of libcapnp if it's not detected

buildutils/vers.cpp

@@ -0,0 +1,9 @@
+// check libcapnp version
+
+#include <stdio.h>
+#include "capnp/common.h"
+
+int main(int argc, char **argv){
+    fprintf(stdout, "vers: %d.%d.%d\n", CAPNP_VERSION_MAJOR, CAPNP_VERSION_MINOR, CAPNP_VERSION_MICRO);
+    return 0;
+}